Well, Mikoto was allowed to go through a reasonably lengthy (tsun)dere-fication process to where she is now. As well as Biribiri Vision. ( ah~ the benefits of rarely seeing Touma, thus mainly seeing only his good points~ )
Seiri feels refreshing because of how she is at this point early on, but she's also the kind of character that's best at low doses. If there's too much of her ( in the context of the Index 'plots' ), even her Iron Wall will break down before the T-Virus.
After all, excessive story exposure means prolonged exposure to all symbiotic variants of the T-Virus.